Sizing an inverter: pure sine vs modified sine and why it matters for pumps
An inverter converts DC battery voltage to AC mains voltage. Growing systems need AC for most equipment: water pumps, air pumps, heaters, grow lights. The inverter has to handle the combined load plus the startup surge from motors.
Pure sine wave vs modified sine wave
Modified sine wave (MSW): Produces a stepped approximation of a sine wave. Cheap (relatively affordable). Works fine for resistive loads: heaters, incandescent lights, simple fans. Problems with inductive loads (motors, pumps) and electronic loads (LED drivers, digital timers, variable-speed pumps). AC motors on modified sine run hotter, hum audibly, and lose 10-20% efficiency. LED grow light drivers may flicker, buzz, or fail prematurely. Timer circuits may malfunction.
Pure sine wave (PSW): Produces a clean sine wave identical to grid power. Costs 2-3x more (roughly double to triple the cost). All loads run normally: motors are quiet and efficient, LED drivers are happy, timers work, and sensitive electronics are safe.
For a growing system with pumps, grow lights, and timers, a pure sine wave inverter is the right choice. The price premium is small relative to the total system cost, and the problems MSW causes with pump motors and LED drivers are real, not theoretical.
Continuous vs surge rating
Every inverter has two power ratings:
Continuous (rated) power: What the inverter delivers indefinitely. A "1000W" inverter runs a 1000W load all day.
Surge (peak) power: What the inverter delivers for a few seconds during motor startup. Usually 2x the continuous rating. A "1000W continuous / 2000W surge" inverter handles a motor that draws 1800W during the initial start but settles to 400W running.
The number that matters for sizing is continuous power for the total running load, and surge power for the highest single motor starting while everything else is already running.
Pump inrush current
AC induction motors (the type in most submersible water pumps and air compressors) draw 3-7x their running current during startup. A pump rated at 50W running may draw 200-350W for the first 0.5-2 seconds as the motor spins up.
If the inverter's surge capacity can't handle the inrush, it either shuts down on overcurrent protection (the pump doesn't start) or clips the waveform (the motor starts slowly, runs hot, and the inverter buzzes). Neither is good for the equipment.
Sizing rule: Total running load + largest single motor's startup surge must be below the inverter's surge rating.
Example system:
- Water pump: 30W running, ~150W startup surge
- Air pump: 5W running, ~20W startup surge
- Heater: 200W (resistive, no surge)
- LED grow light: 150W (electronic, minimal surge)
Total running: 385W. Worst-case surge: everything running + water pump starting = 385W running + 150W pump surge = 535W peak.
A 600W continuous / 1200W surge pure sine inverter handles this with margin. A 500W inverter would be tight on continuous and fine on surge; a 1000W inverter has comfortable headroom.
Voltage
Inverters come in 12V, 24V, and 48V input versions, matching the battery bank voltage.
12V: Simple, common, cheap. Fine for systems under 1,000W. Above 1,000W, the DC current gets very high (1,000W at 12V = 83A), requiring thick expensive cables and causing significant cable losses.
24V: The sweet spot for 1,000-3,000W systems. Half the current of 12V at the same power. Cable sizes and losses are reasonable.
48V: Standard for systems above 3,000W. Most commercial off-grid inverter-chargers (Victron, Growatt, EG4) are 48V. Lower current, lower losses, thinner cables.
For a typical growing system drawing 500-2,000W, a 24V or 48V system is practical. 12V works for small setups (under 500W total load) with short cable runs between battery and inverter.
Inverter-chargers
An inverter-charger combines the inverter with a solar charge controller and (often) grid charging in one unit. Brands like Victron MultiPlus, Growatt SPF, and EG4 make all-in-one units that handle solar input, battery management, AC output, and grid backup in a single box.
For a dedicated growing-system installation, an inverter-charger simplifies wiring and reduces the number of components. For a DIY build pieced together from separate components (separate solar charge controller, separate inverter), buying each unit individually gives more flexibility but more wiring complexity.
The inverter calculator on this site helps match inverter capacity to the system load, accounting for motor surge and the continuous running total. The system cost calculator includes inverter costs in the full system estimate.